Power supplies for computer systems are typically designed to meet the maximum power demand of the load, such as the computer systems receiving power from the power supplies, and for redundancy. While these factors are important in power supply design, energy efficiency is an equally important factor that is usually not given the same weight as other factors when designing power supplies.
FIG. 5 illustrates a conventional power supply 500 modeled as a black box with power entering the black box (input power) and conditioned power (output power) exiting the black box. Conditioning may include alternating current (A/C) or direct current (D/C) conversions (e.g., AC/AC, AC/DC, DC/DC, etc.), and the like. Ideally, there would be no losses between the input power and the output power. However, in reality, losses occur during the conditioning, typically as heat dissipation. Efficiency of a power supply may be measured as the ratio of output power over input power. For example, if 100 Watts (W) are input to the power supply 500 and 75 W of conditioned power exits the power supply 500, the power supply has 75% efficiency. 25 W of heat may be dissipated by the power supply 500. The efficiency of a power supply is usually provided by a manufacturer, but may also be measured,
FIG. 6 illustrates an exemplary efficiency curve for an AC/DC power supply input power at 200 Volts (V) and 60 Hertz (Hz). The efficiency curve of FIG. 6 may be provided by the power supply manufacturer or determined through power measurements. Referring to the efficiency curve of FIG. 6, the power supply is approximately most efficient (e.g., approximately 80%) with a power output between 400 W and 450 W. Conventional power systems for computer systems use at least two power supplies for redundancy, whereby each power supply is operable to meet the power demand of the computer systems unilaterally in case of failure of one of the power supplies. However, for the majority of their operation, both power supplies are operational and are usually designed to split the load. Therefore, if the computer systems demand 400 W, the power supplies each only operate at approximately 74% efficiency (e.g., each power supply supplying an output power of approximately 200 W at 74% efficiency per power supply). If three power supplies are used, each of the power supplies only operates at approximately 64% efficiency. Therefore, conventional power systems for computer systems typically sacrifice efficiency for other factors (e.g., redundancy), which leads to increased energy costs.
Power factor is another important characteristic typically considered when designing a power supply since the power factor impacts the sizing of the electrical wires and equipment that supply energy to the power supply and the cost of electricity. Power factor is the ratio of real power over apparent power (see Equation 1).Power Factor=real power/apparent power  Equation (1)
Power factor is based on the type of load on the power supply. A purely resistive load has a power factor of 1, which is ideal, because the real power is equal to the apparent power. However, for non-purely resistive loads, real power is less than apparent power, leading to power factors less than 1. As the difference between apparent and real power increases (i.e., with smaller power factors), more current must be generated by the power source in order to deliver a specific amount of real power to the load. For example, in a system with a power factor of 0.5, to deliver 100 W of real power (10 Amps at 10 Volts) requires the power source to provide 20 Amps at 10 Volts. In a load with a sinusoidal voltage and current, the real power is equal to the product of the RMS input voltage (V), input current (I), and cos(Φ), where cos(Φ) is the phase angle between the voltage and the current. Cos(Φ) is the power factor.
The difference between apparent and real power impacts the cost of the electrical equipment that provides power to a computer system power supply, because all the electrical components upstream of the power supply must be sized for a higher current. In addition, because all components dissipate some heat when current passes through them, higher currents translate into greater power wastage. To offset this cost and the cost of the greater power wastage, electrical utilities charge, in general, more for electricity provided to lower power factor loads.
Typically, power supplies for computer systems may have a power factor between 0.6 and 0.8. A poor power factor may be the result of a large amount of reactive power caused by an inductive load. The output power of a power supply can be modeled based on power factor and efficiency (see Equation 2).Output Power=efficiency*power factor*apparent power  Equation (2)
FIG. 7 illustrates an exemplary power factor curve for the power supply having the efficiency curve shown in FIG. 6. The power factor curve shown in FIG. 7 may be provided by a manufacturer (e.g., based on a predetermined load) or may be calculated from power measurements. Based on this power factor curve, a higher power factor is achieved generally as output power of the power supply is increased. Power factor correction circuits are generally used to improve factor. However, power factor is typically not considered when optimizing the efficiency of a power supply or power system.